9000 x g supernatant (S-9) of liver homogenates from mammals is commonly used in the in vitro activation system to study the mutagenicity of chemical carcinogens which require metabolic activation for their biological activities. Several studies has shown that the mutagenic activity of these compounds can be enhanced if S-9 is from animals pretreated with enzyme inducers such as phenobarbital (PB), 3-methylcholanthrene (3-MC), or Arochlor 1254. Among these inducers, Arochlor 1254 appears to be less specific and can enhance a variety of enzyme activities. It has been shown, however, that Arochlor is carcinogenic. Studies using the Salmonella plate incorporation test system strains TA98 and TA1535 are being conducted to determine whether combined injection of beta-naphthoflavone and PB can be used as a safe substitute for Arochlor as inducers in the in vitro activation system.